[mlir][vector] Use optional for outerproduct accumulator instead of variadic

This was introduced before the Optional directive and uses Variadic, but
it's really optional.

Reviewed By: nicolasvasilache, benmxwl-arm, dcaballe

Differential Revision: https://reviews.llvm.org/D159259

Author: c-rhodes

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -897,7 +897,7 @@ def Vector_OuterProductOp :
                 TCresVTEtIsSameAsOpBase<0, 1>>,
     DeclareOpInterfaceMethods<MaskableOpInterface>]>,
     Arguments<(ins AnyVector:$lhs, AnyType:$rhs,
-               Variadic<AnyVector>:$acc,
+               Optional<AnyVector>:$acc,
                DefaultValuedAttr<Vector_CombiningKindAttr, "CombiningKind::ADD">:$kind)>,
     Results<(outs AnyVector)> {
   let summary = "vector outerproduct with optional fused add";
@@ -961,9 +961,9 @@ def Vector_OuterProductOp :
       return getRhs().getType();
     }
     VectorType getOperandVectorTypeACC() {
-      return getAcc().empty()
-        ? VectorType()
-        : ::llvm::cast<VectorType>((*getAcc().begin()).getType());
+      return getAcc()
+        ? ::llvm::cast<VectorType>(getAcc().getType())
+        : VectorType();
     }
     VectorType getResultVectorType() {
       return ::llvm::cast<VectorType>(getResult().getType());

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -2756,7 +2756,7 @@ void OuterProductOp::build(OpBuilder &builder, OperationState &result,
 
 void OuterProductOp::print(OpAsmPrinter &p) {
   p << " " << getLhs() << ", " << getRhs();
-  if (!getAcc().empty()) {
+  if (getAcc()) {
     p << ", " << getAcc();
     p.printOptionalAttrDict((*this)->getAttrs());
   }

mlir/lib/Dialect/Vector/Transforms/LowerVectorContract.cpp

@@ -1128,7 +1128,7 @@ class OuterProductOpLowering : public OpRewritePattern<vector::OuterProductOp> {
     VectorType resType = op.getResultVectorType();
     Type eltType = resType.getElementType();
     bool isInt = isa<IntegerType, IndexType>(eltType);
-    Value acc = (op.getAcc().empty()) ? nullptr : op.getAcc()[0];
+    Value acc = op.getAcc();
     vector::CombiningKind kind = op.getKind();
 
     // Vector mask setup.